1. Field of the Invention
This invention relates generally to fault isolation detection system and, more particularly, to a fault isolation detection system for a multi-stack fuel cell system.
2. Discussion of the Related Art
Hydrogen is a very attractive source of fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of, hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle. Many fuels cells are combined in a stack, and several stacks are electrically coupled in series, to generate the desired power.
FIG. 1 is a schematic plan view of a fuel cell stack 10. The stack 10 is enclosed in a housing 12. Several fuel cell stacks are configured in a multi-stack system. A coolant loop flows within the housing 12 to cool the stacks during operation. The stack 10 includes a positive terminal (anode) 14 and a negative terminal (cathode) 16 that are electrically coupled to the respective terminals of the stacks within the housing 12. The resistance Rn identifies a negative conductive path between the negative terminal 16 and ground through the coolant loop, and the resistance Rp identifies a positive conductive path between the positive terminal 14 and ground through the coolant loop. It is known that the resistance Rn will be significantly greater than the resistance Rp.
Fault isolation detection systems are sometimes employed in electrical systems to isolate persons from the electrical circuit therein. Fault isolation detection systems provide fault detection so that if the person does come in contact with high voltage portions of the circuit, the system will detect the event and open a circuit within a few milliseconds to prevent the person from being injured. Fuel cell systems are one electrical system that employ fault isolation detection systems to prevent a person from being electrocuted by the system, such as coming in contact with the positive terminal 14 or the negative terminal 16 and ground, such as the vehicle chassis. The known isolation detection systems for a fuel cell system prevent a current feed-back path from the negative terminal 16 to the positive terminal 14, and vice versa. For example, resistors are provided to limit the current flow between the positive terminal 14 and ground and the negative terminal 16 and ground.
FIG. 2 is a schematic diagram of a classical electrical isolation system 20. A voltage source 22 provides the voltage potential available to the system 20. The system 20 includes a voltage divider 24, having resistors R1 and R2, where R1 equals R2. The resistor R1 is a current limiting device between the positive terminal of the voltage source 22 and ground, and the resistor R2 is a current limiting device between the negative terminal of the voltage source 20 and ground. A resistor R3 is electrically coupled to the voltage divider 24 between the resistors R1 and R2 and ground, as shown. The values of the resistors R1, R2 and R3 are selected to provide a balanced current loop, as discussed below, when a no-fault condition exists. The resistors R1 and R2 typically have a value of 500 k ohms or higher and the resistor R3 has a value of approximately 2.5 k ohms.
The resistor R3 is used to detect an isolation failure or fault condition. A voltage drop is monitored across the resistor R3, where the value of the voltage drop when the system 20 is isolated is very small. When a fault condition occurs, the voltage drop across the resistor R3 increases, and peripheral circuitry, such as an operational amplifier 26, measures the voltage increase and provides an output signal. For example, if a person simultaneously touches the positive or negative terminal of the source 22 and ground, the electrical balance provided by the resistors R1 and R2 is upset, and an increased voltage-drop occurs across R3. The voltage drop is measured by the amplifier 26 to indicate a fault condition. The resistor R4 represents the person simultaneously touching the positive terminal of the voltage source 22, and ground that unbalances the voltage divider 24 to provide the voltage drop across the resistor R3. The voltage drop across the resistor R3 is an indication of the amount of current that is traveling through the new resistance path, i.e., the person. Circuitry can be electrically coupled to the output of the amplifier 26 to open a circuit and remove the current flow before the person is injured.
When detecting isolation faults in a single stack fuel cell system, classical isolation techniques can be employed. FIG. 3 is a schematic diagram of a circuit 28 showing the classical isolation technique in FIG. 2, and including the resistances Rp and Rn identified above. The combination of the resistors R1, R2, Rp, and Rn define a Wheatstone bridge. In this design, the resistors R1 and Rp will have the same value and the resistors R2 and Rn will have the same value. By determining the ratio of the resistances Rn/Rp in the single stack system, the resistors R1 and R2 can be sized to compensate for the imbalance between the positive and negative conductive paths in the coolant loop. In other words, even though the resistance Rn will be greater than the resistance Rp, the values of the resistors R1 and R2 can be provided accordingly so that when there is a no-fault condition, there is a minimal voltage drop across the resistor R3. Changes in the stack voltage will not affect the ratio of the balanced circuit. If a fault condition does occur where additional resistance is added to one of either Rp or Rn indicating a fault, then the Wheatstone bridge will become unbalanced, and a larger voltage drop will occur across the resistor R3 that can be measured to indicate the fault condition.
The classical fault isolation detection technique discussed above is not adequate for a multi-stack fuel cell system where the several stacks are electrically coupled in series because there are multiple leakage paths within the coolant loop. In other words, because there are several resistive paths between ground and the respective terminal of each stack through the coolant loop, and the voltage of each stack is changing independently of the other stacks, a balanced system cannot be provided by the Wheatstone bridge in FIG. 3 because the ratios between the resistances Rn and Rp will be different for the several stacks.
FIG. 4 is a general schematic diagram of an electrical system 30 for a multi-stack fuel cell system showing the positive resistance Rp and voltage sources V1–V4 identifying the voltages for four stacks in the multi-stack. A terminal 32 is the positive terminal of the multi-stack and a terminal 34 is the negative terminal of the multi-stack. A voltage potential exists between the positive terminal 32 and chassis ground and the negative terminal 34 and chassis ground. A voltage Vs is the voltage potential of the multi-stack between the terminals 32 and 34. The voltage from the voltage sources V1–V4 is varying in time relative to each other. Because the negative resistances Rn1–Rn4 are relatively high compared to the positive resistances Rp1–Rp4 of the stacks, they are negligible for the isolation detection purposes discussed herein, and have been eliminated. The negative resistance Rn for the last stack is shown because a positive resistance Rp does not follow it.
FIGS. 5(a)–5(d) show the electrical system 30 superimposed into four separate circuits 36–42, respectively, where a single voltage source 44 is positioned at each location of the voltage sources V1–V4.
FIG. 6 is a schematic diagram of a fault isolation detection circuit 46 including the electrical system 30 and a voltage divider 48 made up of resistors R5, R6 and R7 to provide isolation detection of the type discussed above for a multi-stack fuel cell system. The value of the resistors R5, R6 and R7 can be selected so that the circuit 46 is electrically balanced for a particular output voltage for each stack. However, because there are several voltage sources in a multi-stack system, there are multiple current paths that can create an imbalance in the isolation circuit 46. The changing voltages cause varying voltage drops across the resistances Rp1–Rp4 and Rn, thus the voltage drop across the resistor R5 changes dramatically. This identifies the problem in that the stacks are still considered isolated, but the detection circuit 46 would show a fault condition and shut the system down.